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& \\
\multicolumn{2}{|c|}{\LARGE\bf THE\hspace*{1cm}STAR\hspace*{1cm}FORMATION\hspace*{1cm}NEWSLETTER} \\ [0.3cm]
\multicolumn{2}{|c|}{\large\em An electronic publication dedicated to early stellar evolution and molecular clouds} \\ [0.3cm]
{\hspace*{0.8cm} No. 51 --- 14 Dec 1996 } & \multicolumn{1}{r|}{Editor: Bo Reipurth (reipurth@eso.org)\hspace*{0.8cm}} \\ [-0.1cm]
& \\ \hline
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\vspace*{1cm}
%\begin{center}
%{\Large\em From the Editor}
%\end{center}
%\vspace*{0.6cm}
{\large\bf{ Collapse and Fragmentation of Molecular Cloud Cores.
V. Loss of Magnetic Field Support }}
{\bf{ Alan P. Boss }}
{DTM, Carnegie Institution of Washington,
5241 Broad Branch Road, NW, Washington, DC 20015-1305, USA}
{E-mail contact: boss@dtm.ciw.edu}
{The fragmentation mechanism has been quite successful at providing
an explanation for the formation of binary stars during the collapse
phase of dense cloud cores. However, nearly all fragmentation calculations
to date have ignored the effects of magnetic fields, whereas magnetic
fields are generally regarded as the dominant force in molecular clouds.
Here we present the first three dimensional, radiative hydrodynamical
models of the collapse and fragmentation of dense molecular cloud
cores, including the effects of magnetic fields and ambipolar diffusion.
Starting from a prolate, Gaussian cloud that would collapse and fragment
in the absence of magnetic fields (a thermally supercritical cloud),
we introduce sufficient magnetic field support (through the magnetic
field pressure, $B^2/8\pi$, with $B = B_o (\rho/\rho_o)^\kappa$)
to ensure a magnetically subcritical (stable) cloud. The
effects of ambipolar diffusion are then simulated by reducing the
magnetic pressure scaling factor ($B_o$) over a specified time interval
($= t_{AD}$), leading to a magnetically supercritical cloud and
collapse. When $t_{AD}$ is about 10 free fall times or less,
fragmentation into a binary can still occur; for longer $t_{AD}$,
fragmentation is stifled by the slow onset of collapse.
While binary fragmentation is
possible in a magnetically-supported cloud, the outcome depends critically
on the time scale for ambipolar diffusion.
Estimates of the time scale for ambipolar diffusion in dense clouds
turn out to be roughly equal to this critical value, suggesting that
whether fragmentation occurs or not depends on the detailed
characteristics of the cloud and its collapse.}
{Accepted by Astrophys. J.}
\vspace*{0.5cm}
{\large\bf Forbidden Emission Lines in Herbig Ae/Be Stars}
%% Here comes the author(s) of the paper, please indicate within $^...$
%% the number which corresponds to the institute of each author.
{\bf{Myles T.P.\ Corcoran \& Thomas P.\ Ray}}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
{$^1$School of Cosmic Physics, Dublin Institute for Advanced Studies, 5
Merrion Square, Dublin 2, Ireland\\
$^2$Service d'Astrophysique, CEA, Orme des Merisiers, 91191 Gif-sur-Yvette, France}
{Email contact: corcoran@discovery.saclay.cea.fr}
%% Within the following brackets you place your text:
{The absence of high velocity redshifted forbidden lines in classical
T-Tauri stars (Appenzeller et al.\ 1984; Edwards et al.\ 1987) has
long be taken as evidence of opaque circumstellar disks: Disks which
occlude the receding component of the stellar wind or outflow,
allowing only the blueshifted emission to be observed. There has been
some controversy in the literature recently as to whether a disk model
is appropriate to the higher mass counterparts of the T-Tauri stars:
the Herbig Ae/Be stars. With this controversy in mind, and a search
for such occluding effects, we present part of a comprehensive study
of 56 Herbig Ae/Be stars, 28 of which are observed to possess
detectable [OI]$\lambda$6300 emission. It was found that those stars
with [OI]$\lambda$6300 emission can be divided into four distinct
groups as determined by line profiles and velocities. Roughly 15\% (4)
of the sample show both high and low velocity blueshifted forbidden
emission lines reminiscent of the line profiles of classical T-Tauri
stars with extended outflows. Of the three remaining groups, the first
shows low velocity blueshifted emission with centroid velocities in
the range -55 kms$^{\rm -1}$ $\leq$ v$_c \leq$ -10 kms$^{\rm -1}$ (14
stars), the second unshifted ($\mid$v$_c$$\mid \leq$ 5 kms$^{\rm -1}$)
symmetrical forbidden emission lines (7 stars) and the third group of
3 stars low velocity (10 kms$^{\rm -1}$ $\leq$v$_c$$\leq$ 15 kms$^{\rm
-1}$) redshifted emission. No Herbig Ae/Be star was found to possess
strongly redshifted forbidden line emission. The clear tendency
towards blueshifted velocities not only implicitly suggests the
presence of occluding disks around these stars but there also appears
to be a link between the degree of embeddedness and the amount of
forbidden line shift. An evolutionary effect may be responsible in the
sense that, as the star becomes less enshrouded, the high velocity
(jet) component of the forbidden line emission disappears first,
followed by a decrease in the velocity of the low velocity component
and finally by its disappearance altogether. The low velocity
forbidden line emission is most likely a disk wind, the line profile
being broadened as a result of the rotation of the disk. It is found
that the line widths of the low velocity forbidden line emission are
broader than those found in the classical T-Tauri stars. There is also
evidence of acceleration in the outflow, traced by an increase in the
blueshifted velocities from the [OI]$\lambda$6300 to the
[SII]$\lambda\lambda$6717/6731 lines.}
% Here you write which journal accepted your paper, for example:
{ Accepted by Astronomy \& Astrophysics }
\vspace*{0.5cm}
%% Between these brackets you write the title of your paper:
{\large\bf
{Intercloud Structure in a Turbulent, Fractal Interstellar Medium}}
%% Here comes the author(s) of the paper, please indicate within $^...$
%% the number which corresponds to the institute of each author.
{\bf{Bruce Elmegreen}}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
{IBM T.J. Watson Research Center, PO Box 218 Yorktown Heights NY
10598, USA}
%% Here you may write the e-mail address of one or more of the authors
%% who will act as contact person for preprint requests etc, for example:
{E-mail contact: bge@watson.ibm.com}
%% If you use any personal Latex commands in your abstract, please include
%% their definitions here.
%% Within the following brackets you place your text:
{Pervasive turbulence and fractal structure in the interstellar gas
imply the existence of large holes and gaps, filling $\ge80$\% of the
volume, that may be identified with the intercloud medium (ICM). Such
an ICM needs no supernovae or other localized sources for clearing;
extensive supernova clearing seems unlikely anyway on both observational
and theoretical grounds. Fractal clouds produce fractal ionization
zones (FIZ) in which O-star radiation can travel at least twice as far
as in a standard Stromgren sphere, and they contain extensive holes
covering $\sim50$\% of the sky through which this radiation can reach
the Galactic halo.
Clouds in a fractal medium are not randomly distributed like standard
clouds in the conventional model; they are highly clumped and clustered.
If most of the interstellar gas is in such fractal cloud complexes, then
there are on average 3 per kiloparsec on a line of sight. These 3 alone
produce the observed 8 "standard-cloud" absorption lines per kiloparsec
by placing $\sim5$ absorption features on each occupied line of sight
through a cloud and none on the unoccupied lines of sight. The mean
length of an unoccupied line of sight is $\sim600$ pc.}
% Here you write which journal accepted your paper, for example:
{Accepted by Astrophys. J., scheduled for Mar 1 issue }
%% If preprints are available on the WWW you can give the web
%% direction here.
\vspace{0.5cm}
%% Between these brackets you write the title of your paper:
{\large\bf{The Multiplicity of Pre-Main Sequence Stars in Southern Star Forming Regions}}
%% the number which corresponds to the institute of each author.
{\bf{A. M. Ghez$^1$, D. W. McCarthy, Jr.$^2$, J. L. Patience$^1$,
T. L. Beck$^{1,3}$ }}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
$^1$ {Department of Physics and Astronomy, University of
California, Los Angeles, CA 90095-1562, USA} \\
$^2$ {Steward Observatory, University of Arizona, Tucson, AZ 85721, USA} \\
$^3$ {Astronomy Program, State University of New York, Stony Brook, NY
11794-2100, USA}
%% Here you may write the e-mail address of one or more of the authors
%% who will act as contact person for preprint requests etc, for example:
{E-mail contact: ghez@astro.ucla.edu}
%% If you use any personal Latex commands in your abstract, please include
%% their definitions here.
%% Within the following brackets you place your text:
{High resolution studies of young stars in the star forming regions of Taurus
and Ophiuchus have revealed a large population of multiple star systems. To
test how applicable this earlier result is for other star forming regions, we
have carried out a K[2.2 $\mu m$] band multiplicity survey
of pre-main sequence stars located in the dark cloud complexes Chameleon, Lupus,
and Corona Australis. This survey, which was conducted with both
speckle and direct imaging techniques, covers a binary star
separation range of 0.$''$1 to 12$''$ (15 - 1,800 AU) and identifies 25
companion stars of which 9 are new detections. The companion star fraction over
the separation range covered by this survey is estimated to be 0.52 $\pm$ 0.11,
in agreement with Taurus (0.58 $\pm$ 0.08) and Ophiuchus (0.50 $\pm$ 0.12).
A comparison of the direct imaging
portion of this survey with Reipurth \& Zinnecker's (1993) optical
multiplicity study reveals that 4\% of the overlap sample have
"infrared companions", companions too red to be detected at optical
wavelengths. This suggests that infrared surveys will systematically
measure a slightly higher companion star fraction compared to optical surveys.
The result of combining all K-band surveys of dark cloud
complexes, which cover the separation range 15 - 1800 AU, shows a
factor of two excess of the companion star fraction for young stars compared to
that for the solar-type stars in the solar neighborhood (0.54 $\pm$ 0.06 vs.
0.26 $\pm$ 0.04).}
% Here you write which journal accepted your paper, for example:
{ Accepted by Ap. J. }
%% If preprints are available on the WWW you can give the web
%% direction here.
\newpage
%% Between these brackets you write the title of your paper:
{\large\bf{X-Ray Ionization of Protoplanetary Disks}}
%% Here comes the author(s) of the paper, please indicate within $^...$
%% the number which corresponds to the institute of each author.
{\bf{A.E. Glassgold$^1$, J. Najita$^2$, and J. Igea$^1$ }}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
$^1$ {Physics Department, New York University, 4 Washington Place, New York, NY
10003, USA} \\
$^2$ {Center for Astrophysics, 60 Garden Street, Cambridge, MA 01238, USA}
%% Here you may write the e-mail address of one or more of the authors
%% who will act as contact person for preprint requests etc., for example:
{E-mail contact: glassgol@windy.physics.nyu}
%% Within the following brackets you place your text:
{ In the light of new observations of star forming regions by {\it ASCA}
and {\it ROSAT}, we assess the ability of young stars to ionize their own
circumstellar disks with stellar coronal X-rays. Although stellar winds may
absorb soft X-rays, hard X-rays can penetrate to large column densities and,
until they are absorbed, produce ionization rates greater than standard
estimates for Galactic cosmic rays. As in previous studies of the external
ionization of protoplanetary disks by cosmic rays, we find that X-ray
ionization produces a surface layer that is well-coupled to disk magnetic
fields at $\sim$ AU distances. The properties of the surface layer depend
on the characteristics of the X-ray source, and thus on the evolutionary
status of the central star. Even if Galactic cosmic rays are efficiently
excluded by magnetized winds, stellar X-ray irradiation alone may provide
sufficient ionization for disks to accrete via the Balbus-Hawley instability.
The resulting vertically-layered structure of disks at $\sim$ AU distances (a
well-coupled surface layer overlying a poorly-coupled, deeper layer) may
lead to divergent dynamical evolution in the two regions. While accretion in
the surface layer contributes to the buildup of the mass of the star, the
quiescent conditions in the poorly-coupled, deeper layer appear to be conducive
to the formation of planets.}
% Here you write which journal accepted your paper, for example:
{Accepted by Astrophysical Journal}
\vspace{0.5cm}
%% Between these brackets you write the title of your paper:
{\large\bf{The Stability of Radiatively Cooling Jets: I. Linear Analysis}}
%% Here comes the author(s) of the paper, please indicate within $^...$
%% the number which corresponds to the institute of each author.
{\bf{Philip E. Hardee$^{1}$ and James M. Stone$^2$ }}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
$^1$ {Department of Physics \& Astronomy, The University of Alabama,
Tuscaloosa, AL 35487, USA hardee@venus.astr.ua.edu} \\
$^2$ {Department of Astronomy, The University of Maryland, College Park,
MD 20742, USA jstone@astro.umd.edu}
%% Here you may write the e-mail address of one or more of the authors
%% who will act as contact person for preprint requests etc., for example:
{E-mail contact: jstone@astro.umd.edu}
%% Within the following brackets you place your text:
{The results of a spatial stability analysis of a two-dimensional slab
jet in which optically thin radiative cooling is dynamically important
are presented. We study both magnetized and unmagnetized jets at
external Mach numbers of 5 and 20. We model the cooling rate using two
different cooling curves: one appropriate to interstellar gas, and the
other to photoionized gas of reduced metallicity. Thus, our results
will be applicable to both protostellar (Herbig-Haro) jets, and optical
jets from active galactic nuclei.
We present analytical solutions to the dispersion relations in useful
limits and solve the dispersion relations numerically over a broad
range of perturbation frequencies. We find that the growth rates and
wavelengths of the unstable Kelvin-Helmholtz (K-H) modes are
significantly different from the adiabatic limit, and that the form of
the cooling function strongly affects the results. In particular, if
the cooling curve is a steep function of temperature in the
neighborhood of the equilibrium state, then the growth of K-H modes is
reduced relative to the adiabatic jet. On the other hand, if the
cooling curve is a shallow function of temperature, then the growth of
K-H modes can be enhanced relative to the adiabatic jet by the increase
in cooling relative to heating in overdense regions. Inclusion of a
dynamically important magnetic field does not strongly modify the
important differences between an adiabatic and cooling jet, provided
the jet is highly supermagnetosonic and not magnetic pressure
dominated. In the latter case, the unstable modes behave more like the
transmagnetosonic magnetic pressure dominated adiabatic limit. We also
plot fluid displacement surfaces associated with the various waves in a
cooling jet in order to predict the structures which might arise in the
nonlinear regime. This analysis predicts that low frequency surface
waves and the lowest order body modes will be the most effective at
producing observable features in the jet.}
% Here you write which journal accepted your paper, for example:
{ Accepted by Ap.J. }
%% If preprints are available on the WWW you can give the web
%% direction here.
\newpage
%% Between these brackets you write the title of your paper:
{\large\bf{S255-2, The Formation of a Stellar Cluster}}
%% Here comes the author(s) of the paper, please indicate within $^...$
%% the number which corresponds to the institute of each author.
{\bf{Eric M. Howard$^{1,2}$, Judith L. Pipher$^2$ \ and William J.
Forrest$^2$}}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
$^1$ {Department of Physics and Astronomy, University of Massachusetts,
LGRT, Amherst, MA 01003-4525, USA}
\\
$^2$ {Department of Physics and Astronomy, University of Rochester,
Rochester, NY 14627-0171, USA}
%% Here you may write the e-mail address of one or more of the authors
%% who will act as contact person for preprint requests etc, for example:
{E-mail contact: ehwd@kutath.phast.umass.edu}
%% If you use any personal Latex commands in your abstract, please include
%% their definitions here.
%% Within the following brackets you place your text:
{As part of an
ongoing study of high mass star formation regions, we have imaged the
S255-2 triple HII region in near-infrared broadband wavelength bands J
(1.23 $\mu$m), H (1.65 $\mu$m), K (2.23 $\mu$m), and at 3.3 $\mu$m. We
have also obtained images in the $Br\gamma$ and $Br\alpha$ hydrogen
recombination lines, in the H$_2$ v = 1 $\rightarrow$ 0 S(1) line and in
the 3.29 $\mu$m dust feature emission band.
The region consists of a
circular core of stars and YSOs, as well as nebulosity, and a more diffuse
stellar cluster. The Brackett line emission from the region is at least a
factor of ten greater than the value estimated from the radio continuum
flux density, assuming case B recombination. The strongest source of
$Br\alpha$ line emission is IRS1b and this source appears to be an
ionizing wind source. The central core region contains a narrow band of
$Br\gamma$ and $H_2$ emission that we postulate is an ionized jet. The
3.29 $\mu$m and $H_2$ emission are found in a bubble-like region that
overlaps and extends beyond the $Br\alpha$ and $Br\gamma$ emission into
the photodisociation region (PDR). S255-2 appears to be a young cluster
of stars still in the process of forming.}
% Here you write which journal accepted your paper, for example:
{ Accepted by ApJ.}
%% If preprints are available on the WWW you can give the web
%% direction here.
{preprints are available from
http://kutath.phast.umass.edu/~ehwd/papers/papers.html}
\vspace{0.5cm}
%% Between these brackets you write the title of your paper:
{\large\bf{Kinematic Studies of Herbig-Haro Objects in the Orion Nebula}}
%% Here comes the author(s) of the paper, please indicate within $^...$
%% the number which corresponds to the institute of each author.
{\bf{ Xihai Hu}}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
{Department of Space Physics and Astronomy, Rice University, Houston, TX 77251, USA}
%% Here you may write the e-mail address of one or more of the authors
%% who will act as contact person for preprint requests etc, for example:
{E-mail contact: hux@spacsun.rice.edu}
%% If you use any personal Latex commands in your abstract, please include
%% their definitions here.
%% Within the following brackets you place your text:
{Kinematic studies of most of the Herbig-Haro objects in M42 have been
carried out. The proper motions of some of these objects have been determined
through HST narrow band filter images taken four years apart;
radial velocities of these objects have been obtained from high resolution
\'{e}chelle spectra taken at the Kitt Peak observatory using the Coud\'{e} Feed
system. The three dimensional
kinematic properties of these objects are presented. Proper interpretations of
the data are discussed.}
% Here you write which journal accepted your paper, for example:
{ Accepted by Astron. J. }
%% If preprints are available on the WWW you can give the web
%% direction here.
{Available on WWW at: http://spacsun.rice.edu/\~\ hux/pub.html}
\vspace{0.5cm}
%% Between these brackets you write the title of your paper:
{\large\bf{The Infrared Companions of T Tauri Stars}}
%% Here comes the author(s) of the paper, please indicate within $^...$
%% the number which corresponds to the institute of each author.
{\bf{ C. D. Koresko$^{1,2}$, T. M. Herbst$^3$, \ and Ch. Leinert$^3$ }}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
$^1$ {McDonald Observatory, University of Texas at Austin, Austin, TX
78712, USA}
\\
$^2$ {Current address: Department of Geological and Planetary Sciences,
Caltech, Pasadena, CA 91125, USA} \\
$^3$ {Max-Planck-Institut f\"ur Astronomie, K\"onigstuhl 17, D--69117
Heidelberg, Germany}
%% Within the following brackets you place your text:
%% {Note: Must verify that I have the final version before submitting this}
The Infrared Companions (IRCs) associated with several normal low-mass
pre-main sequence (T Tauri) stars pose an interesting problem for theories
of binary star formation. The IRCs have very low infrared color
temperatures and large infrared excesses which have led observers to
suggest that they may be less evolved objects such as protostars. This
paper presents an attempt to understand the IRCs as a class by examining a
broad range of observations and applying simple arguments and models. We
propose that the IRCs may represent relatively normal young low-mass stars
experiencing episodes of enhanced circumstellar extinction, possibly due
to rapid accretion of disk material perturbed by their gravitational
influence at aphelion or perihelion.
% Here you write which journal accepted your paper, for example:
{ Accepted by Astrophysical Journal, 1997 May 1 issue }
Preprints available at http://gulliver.gps.caltech.edu.
\newpage
%% Between these brackets you write the title of your paper:
{\large\bf{Spectroscopic evidence of mass infall towards an embedded infrared
source in the globule DC~303.8-14.2}}
%% Here comes the author(s) of the paper, please indicate within $^...$
%% the number which corresponds to the institute of each author.
{\bf{ Kimmo Lehtinen}}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
{Observatory, University of Helsinki, T\"ahtitorninm\"aki, P.O.\ Box 14,
00014, Helsinki, Finland}
%% Here you may write the e-mail address of one or more of the authors
%% who will act as contact person for preprint requests etc, for example:
{E-mail contact: kimmo.lehtinen@helsinki.fi}
%% If you use any personal Latex commands in your abstract, please include
%% their definitions here.
%% Within the following brackets you place your text:
{We present millimeter molecular line observations towards the embedded
infrared source IRAS~13036-7644, located in the globule DC~303.8-14.2.
The CS($J$=2--1) lines show asymmetrical profiles characteristic of a
cloud undergoing gravitational collapse, i.e.\ double-peaked profiles
with a brighter blue-shifted component. In contrast the
HCO$^{+}$($J$=1--0) line shows towards the IRAS source a double-peaked
profile with a brighter red-shifted component, and a velocity gradient
across the cloud. The hyperfine components of HCN($J$=1--0)
transition show both kinds of double-peaked profiles, and a velocity
gradient. We interpret these results as a simultaneous infall of the
dense gas in the core and a non-collapsing envelope.
The IRAS source drives a bipolar molecular outflow detected in the
$^{12}$CO($J$=1--0) transition.}
% Here you write which journal accepted your paper, for example:
{ Accepted by Astronomy \& Astrophysics }
%% If preprints are available on the WWW you can give the web
%% direction here.
\vspace{0.5cm}
%% Between these brackets you write the title of your paper:
{\large\bf{On Protostellar Disks in Herbig Ae/Be Stars}}
%% Here comes the author(s) of the paper, please indicate within $^...$
%% the number which corresponds to the institute of each author.
{\bf{Anatoly Miroshnichenko, \v{Z}eljko Ivezi\'{c} \ and Moshe Elitzur}}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
{Department of Physics and Astronomy, University of Kentucky, Lexington, KY
40506-0055, USA}
%% Here you may write the e-mail address of one or more of the authors
%% who will act as contact person for preprint requests etc, for example:
{E-mail contact: moshe@pa.uky.edu}
%% If you use any personal Latex commands in your abstract, please include
%% their definitions here.
%% Within the following brackets you place your text:
{The spectral shape of IR emission from Herbig Ae/Be stars has been invoked as
evidence for accretion disks around high-mass protostars. Instead, we present
here models based on spherical envelopes with $r^{-1.5}$ dust density profile
that successfully explain the observed spectral shapes. The spectral energy
distributions (SEDs) of eight primary candidates for protostellar disks are
fitted in detail for all wavelengths available, from visual to far IR. The
only envelope property adjusted in individual sources is the overall visual
optical depth, and it ranges from 0.3 to 3. In each case, our models properly
reproduce the data for both IR excess, visual extinction and reddening. The
success of our models shows that accretion disks cannot make a significant
contribution to the radiation observed in these pre-main sequence stars.}
% Here you write which journal accepted your paper, for example:
{ Accepted for Astrophys. J. Letters, 475; January 20, 1997}
%% If preprints are available on the WWW you can give the web
%% direction here.
http://xxx.lanl.gov/abs/astro-ph/9611061
\vspace{0.5cm}
%% Between these brackets you write the title of your paper:
{\large\bf{Infrared Images and Millimeter Data of Cold Southern IRAS Sources}}
%% Here comes the author(s) of the paper, please indicate within $^...$
%% the number which corresponds to the institute of each author.
{\bf{ M. Osterloh$^1$, Th. Henning$^1$ \ and R. Launhardt$^1$ }}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
$^1$ {Max Planck Society, Research Unit ``Dust in Star Forming Regions'',
Schillerg\"a\ss chen 2-3, D-07745 Jena,
Germany}
%% Here you may write the e-mail address of one or more of the authors
%% who will act as contact person for preprint requests etc., for example:
{E-mail contact: osterloh@fred.astro.uni-jena.de}
%% Within the following brackets you place your text:
{
We present near-infrared (H,K$'$), CO (2-1), CS (2-1) and 1.3 millimeter
continuum
data for 31 southern objects ($\delta(1950)\le$10$^{\circ}$) known to have
extremely
red IRAS colors (F$_{\nu}$(100$\mu$m)$>$F$_{\nu}$(60$\mu$m)$>$
F$_{\nu}$(25$\mu$m)$>$20$\times$F$_{\nu}$(12$\mu$m)).
The data are meant to help reveal
new, very young stellar objects.
K$'$-band near-infrared counterparts to the IRAS point sources
are detected in 22 of 25 good K$'$ images. Most K$'$ counterparts are
multiples.
18 of 21 objects were detected in CS, implying the presence of dense gas.
Completing the set of CS (2-1) spectra by including the data of
Bronfman {\it et al.} (1996), we still find only 3 non-detections among all
31 objects;
these were also
not detected in K$'$. Wings indicative of outflows are found
in a large fraction (20/30) of
CO spectra.
26 of 31 mm continuum observations were detections and point to the presence
of large amounts of circumstellar matter.
Most objects have 10$^3$ up to 10$^5$ solar luminosities;
we speculate that they contain at least one massive star capable of
producing a compact/ultracompact HII region.
}
% Here you write which journal accepted your paper, for example:
{ Accepted by the Astrophysical Journal }
\newpage
%% Between these brackets you write the title of your paper:
{\large\bf{Post-T Tauri stars: a false problem}}
%% Here comes the author(s) of the paper, please indicate within $^...$
%% the number which corresponds to the institute of each author.
{\bf{Francesco Palla$^{1,2}$ and Daniele Galli$^{1}$}}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
$^1$ {Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze,
Italy} \\
$^2$ {Laboratoire de Radioastronomie, Ecole Normale Superieure, 24 rue
Lhomond, 75005 Paris, France}
%% Here you may write the e-mail address of one or more of the authors
%% who will act as contact person for preprint requests etc, for example:
{E-mail contact: palla@arcetri.astro.it}
%% Within the following brackets you place your text:
{We consider the problem of the apparent lack of old T Tauri stars in
nearby star forming regions in the framework of the standard model of
low-mass star formation.
We argue that the similarity between molecular cloud lifetimes and the
ambipolar diffusion timescale implies that star formation does not take
place instantaneously, nor at a constant rate.
Thus, models based on the assumption of a constant star formation rate
overestimate the predicted number of stars of ages greater than
2-5 Myr. We argue that the probability of finding a large population of old
stars in a star forming region is {\it intrinsically} very small.
It is therefore unlikely that the dispersed X-ray sources
found by ROSAT at large distances from molecular cloud complexes
can be identified with such a population.
We conclude that the post-T Tauri problem is by and large not existent. }
% Here you write which journal accepted your paper, for example:
{ Accepted by The Astrophysical Journal (Letters)}
\vspace{0.5cm}
%% Between these brackets you write the title of your paper:
{\large\bf{On the stability of an accretion disc containing a toroidal
magnetic field: The effect of resistivity}}
%% Here comes the author(s) of the paper, please indicate within $^...$
%% the number which corresponds to the institute of each author.
{\bf{ John C.B. Papaloizou$^1$ \ and Caroline Terquem$^{2,1}$ }}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
$^1$ {Astronomy Unit, School of Mathematical Sciences, Queen Mary
and Westfield College, University of London, Mile End Road,
London E1 4NS, UK} \\
$^2$ {Laboratoire d'Astrophysique, Observatoire de Grenoble,
Universit\'e Joseph Fourier/CNRS, BP 53,
38041 Grenoble Cedex 9, France}
%% Here you may write the e-mail address of one or more of the authors
%% who will act as contact person for preprint requests etc., for example:
{E-mail contact: jcbp@qmw.ac.uk, ct@ucolick.org}
%% If you use any personal Latex commands in your abstract, please include
%% their definitions here.
%% Within the following brackets you place your text:
{We extend a previous study of the global stability of a stratified
differentially rotating disc containing a toroidal magnetic field to
include the effect of a non zero resistivity $\eta.$ We consider the
situation when the disc is stable to convection in the absence of the
magnetic field. The most robust buoyancy driven unstable modes, which
occur when the field is strong enough, have low azimuthal mode number
$m.$ They grow exponentially apparently belonging to a discrete
spectrum. They exist for the dimensionless ratio $\eta /(H^2\Omega) $
smaller than $\sim 10^{-2},$ where $\Omega$ is the angular velocity
and $H$ is the disc semithickness. In contrast the magnetorotational
modes develop arbitrarily small radial scale and show transient
amplification as expected from a shearing sheet analysis. The most
robust modes of this type are local in all directions. Because of
their more global character, the buoyancy driven modes may be
important for the generation of large scale fields and outflows. }
% Here you write which journal accepted your paper, for example:
{ Accepted by M.N.R.A.S.}
%% If preprints are available on the WWW you can give the web
%% direction here.
\vspace{0.5cm}
%% Between these brackets you write the title of your paper:
{\large\bf{Rotating Protostars and Protostellar Disks. I. Equilibrium Models.}}
%% Here comes the author(s) of the paper, please indicate within $^...$
%% the number which corresponds to the institute of each author.
{\bf{ Brian K. Pickett$^1$, Richard H. Durisen$^2$ \ and Robert Link$^2$ }}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
$^1$ {NASA/Ames Research Center, M.S. 245-3, Moffett Field, CA 94035-1000, USA} \\
$^2$ {Department of Astronomy, Indiana University, Bloomington, IN 47405-4201, USA}
%% Here you may write the e-mail address of one or more of the authors
%% who will act as contact person for preprint requests etc, for example:
{E-mail contact: pickett@cosmic.arc.nasa.gov}
%% If you use any personal Latex commands in your abstract, please include
%% their definitions here.
%% Within the following brackets you place your text:
{This paper is part of a series dealing with the structure and dynamic
stability of rotating protostellar cores. As a first step in our
study, we have generated numerical equilibrium models for isentropic,
axisymmetric protostellar cores in rapid rotation. These models
represent endstates for collapse from two different types of initial
pre-collapse cloud conditions, chosen to be reasonable cases for the
formation of low or intermediate-mass stars on the basis of other
theoretical or observational work. Specifically, we consider the
equilibrium cores which would form from the collapse of uniformly
rotating clouds with the density distributions of singular isothermal
spheres and of truncated Gaussian spheres.
The major structural differences between the two sequences are largely
due to their distinct angular momentum distributions. The protostellar
cores which result from singular isothermal initial conditions can be
readily interpreted as slowly rotating stars surrounded by massive,
rotationally-supported disks. A `star' and `disk' are not easily
distinguished for the Gaussian cases, but the outer regions of these
models are typically in rapid, nearly Keplerian rotation. For
reasonable assumptions about parameters, the most rapidly rotating
protostellar cores that we can calculate accurately correspond to
highly flattened disk or star/disk systems of roughly solar mass with
equatorial radii of a few AU's or less. For the protostellar cores
that result from the collapse of singular isothermal spheres, we find
that a significant range in parameter space exists in which
protostellar disks are much smaller than the typical dimensions
usually considered for the solar nebula. These conditions may be
conducive to the formation of relatively compact planetary systems
such as 51 Pegasi. Our axisymmetric star/disk models are fully two
dimensional in the sense that both the vertical disk structure and the
central starlike regions are resolved. These models will be used as a
numerical laboratory for studies of various dynamic processes in
protostellar disks.}
% Here you write which journal accepted your paper, for example:
{ Accepted by Icarus}
%% If preprints are available on the WWW you can give the web
%% direction here.
{http://cosmic.arc.nasa.gov:8082/~pickett/prep.html}
\vspace{0.5cm}
%% Between these brackets you write the title of your paper:
{\large\bf{Kinematics of the Ursa Major Molecular Clouds}}
%% Here comes the author(s) of the paper, please indicate within $^...$
%% the number which corresponds to the institute of each author.
{\bf{ Marc W. Pound$^1$ \ and Alyssa A. Goodman$^2$ }}
%% Here you write your institute name(s) and address(es),
%% the number in $^..$ indicates your author number, for example:
$^1$ {Astronomy Dept., Univ. of California, Berkeley, CA 94720, USA} \\
$^2$ {Astronomy Dept., Harvard University, 60 Garden St., Cambridge, MA 02138, USA}
%% Here you may write the e-mail address of one or more of the authors
%% who will act as contact person for preprint requests etc, for example:
{E-mail contact: pound@teddi.berkeley.edu}
%% If you use any personal Latex commands in your abstract, please include
%% their definitions here.
\def\micron{\ifmmode{\,\mu{\rm m}}\else{\thinspace $\mu$m}\fi}
\def\kms{\ifmmode{{\rm km\,s^{-1}}} \else{\thinspace km\thinspace s$^{-1}$}\fi}
\def\invpc{\ifmmode{\,{\rm pc^{-1}}}\else{\thinspace {\rm pc$^{-1}$}}\fi}
%% Within the following brackets you place your text:
{
We present a kinematic analysis of the atomic and molecular gas in
the Ursa Major molecular clouds. The analysis is based on a new CO survey of
the complex made with linear resolution of 0.05 pc and existing H I observations.
The clouds lie in projection on an expanding shell of material known as the
North Celestial Pole loop. The molecular structure of the complex is dominated by
several long ($>$ 5 pc) filaments, some of which are both extremely straight
and extremely narrow ($$}}}$}
\def\asec {$^{\prime\prime}$} % Arcseconds symbol
\def\mug {\hbox{$\mu$G}} % muG
\def\vlsr {\hbox{${V_{\rm LSR}}$}} % Vlsr
\def\hi {\hbox{H{\hskip0.1em}I}} % HI
\def\hii {\hbox{H{\hskip0.1em}II}} % HII
\def\kms {\hbox{km{\hskip0.1em}s$^{-1}$}} % km/s
\def\v {\hbox{\it V}} % italic V
%% Within the following brackets you place your text:
{DR~21 has been imaged in \hi\ at high resolution (\about5\asec) in
both left and right circular polarization with the VLA. The continuum
emission is nearly totally absorbed with opacities $>$ 5 over about 20
\kms; the FWHM line width is unusually large (\about35 \kms). At
least three components are present: two narrow ones at \vlsr\ = $-5$
\kms\ and +10 \kms\ and a negative-velocity wing whose opacity can be
measured out to \vlsr\ = $-30$ \kms\ in some places. No positive
velocity wing is detected. The midpoint of the negative-velocity wing
varies between $-25$ and $-10$ \kms\ and the full extent is typically
\gsim\ 20 \kms. The wing is detected over most of the \hii\ region
core except in the very southwest corner, near \hii\ region C and in
the northern region around \hii\ region D. The opacity in the wing is
\about0.5 toward the eastern diffuse continuum emission and rises to
\about2 toward the \hii\ core. This negative-velocity \hi\ absorption
is probably associated with the high-velocity outflow seen in CO. The
velocity field of this outflow gas shows a high negative velocity tube
through the center of the \hii\ region with lower negative velocities
near the edge of the tube. The \hi\ gas appears to be accelerating
away from the southwest of the core, from a position close to \hii\
region C. This location for the origin of the outflow is consistent
with that inferred from the CO observations.
Zeeman measurements were made toward the DR~21 compact \hii\ region
core. No magnetic field ($|${\bf B}$|